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Old 9th August 2012, 06:30 PM   #81
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Quote:
Originally Posted by MiiB View Post
I'll watch this, and when the concept is a little more settled, I'll start simulating and add servo-ed housekeeping, but first we must decide the front end and how to get the signal out from there...for me the optimum would be to use cascodede current mirrors to currrent assemble the signal. This gives us the option of adding a filter, We could then buffer using Joarcims N-Fet buffer and servo through the housekeeping, this has proven to be a s strong solution in the paradise, Cant see why it should not be a very good solution here as well...

a question is how to handle the offset center current
I think it would be ideal if there was some kind of feedback that automatically adjusted the current source to compensate for the DC offset of the DAC. The reason is because there isn't a certainty that each PCM1794 DAC will have exactly -6.2mA of DC offset current. I think the offset can vary from one sample to the next. I have not investigated this myself however and it may not be that important after all.
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Old 9th August 2012, 06:55 PM   #82
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Well, this is what I've been working on for a while now. The Lundahl transformer is a special unit with extremely low winding resistance. There is a LP RLC filter in each of the collector circuits. It's basically similar to Dr. Leach's except that instead of coupling capacitors I put in a follower stage. Gain is set by the two resistors R1 and R2, which also control the currents in the common base transistors. The 12V supply is floating.

Since the DAC DC current offset appears on both output pins, the transformer with a center tap cancels it out, as well as providing balanced to single ended conversion. The transformer also cancels some of the distortion from the DAC. In theory, the noise should be low because there is only one I/V converter and no other opamps or stages. I've been working on a circuit board as well. I'm just waiting on money.

The performance in simulation is good in my opinion, but not stellar. Noise is very low, with a S/N of over 130dB (@ 1kHz) in simulation.

Anyway, my 2 cents.
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Last edited by dirkwright; 9th August 2012 at 07:03 PM.
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Old 9th August 2012, 06:56 PM   #83
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I have tested a circuit based on the ideas of Shinja, and it works it only have 50db of cmrr .
but i have to test with better current mirrors.
thanks shinja.
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Old 9th August 2012, 07:07 PM   #84
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transformers do make our lives easier.
Dirkwright How much does it cost?
See if it has a gap, or it will saturate if there is imbalance in currents.
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Old 9th August 2012, 08:49 PM   #85
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That bal- to un-bal problem pops up quite often. On another thread we designed a discrete circuit called XCEN.
It is deliberately very puristic.
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Old 9th August 2012, 08:53 PM   #86
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The XCEN -- Balanced to Single Ended Converter

The one above is one of the early versions.




Patrick
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xen-audio.com
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Old 9th August 2012, 08:55 PM   #87
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Quote:
Originally Posted by smms73 View Post
transformers do make our lives easier.
Dirkwright How much does it cost?
See if it has a gap, or it will saturate if there is imbalance in currents.
It costs about $70 here in the USA. There is no gap as far as I know. It is an output transformer so it's much bigger than a typical input type, so it takes a lot more current imbalance to cause problems. It's good to +24dBu before saturation.
Data sheet is here:
http://www.lundahl.se/pdfs/datash/1517.pdf
If the primary windings are in parallel, DCR is about 4.5 ohms. Likewise, secondary windings in parallel produce very low DCR of about 4.5 ohms. If I could find another transformer with center tapped primary and lower DCR that would be super, but 4.5 ohms is pretty darn low. Lundahl is making noises about making silver wire available on all of their transformers, which will lower DCR a little bit more.

With 7.8mA p-p input, the voltage on the secondary is only about 5mVAC, so the transformer is really just moving current and not producing much voltage. It's not really acting as an I/V converter in other words.
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Old 9th August 2012, 11:58 PM   #88
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Quote:
Originally Posted by Joachim Gerhard View Post
That bal- to un-bal problem pops up quite often. On another thread we designed a discrete circuit called XCEN.
It is deliberately very puristic.
today i was trying to find a solution for that, but unfortunately i have not find a topology elegant, the circuit in post71 dont work like the way i was thinking, its not possible to put ccs I1 in that position , such a stupid mistake. Using a current mirror works, but not that well. so i will leave this road for now.

tomorrow i would like to discuss the transistors we can use, if that's alright with you.
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Old 10th August 2012, 12:26 AM   #89
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dirkwright
the distortion caused by a transformer increase as the ratio between the inductance and dcr in parallel with the impedance of the amplifier, if you have a transformer with zero ohms of dcr, the distortion will be also zero, silver will bring down dcr but will increase the costs. 140 dollars its a lot of money for that, but is your call.
Personaly i use auto transformers in the output of my amplifier , they serve to convert my low impedance speakers (4ohms) to (68, 34 or 16ohms), the highest the load, the less crossover distortion, and they give me peace of mind , as i am always doing mods, and some times i make mistakes, they are the ultimate speaker protection. they ware made by me and they are big, the dcr is very low and as the amplifier has very low output impedance the distortion they produce is also very low.
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Old 10th August 2012, 01:45 AM   #90
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Quote:
Originally Posted by smms73 View Post
dirkwright
the distortion caused by a transformer increase as the ratio between the inductance and dcr in parallel with the impedance of the amplifier,
sorry, is not in parallel is in serie
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